The present invention relates to microelectromechanical devices and associated fabrication methods and, more particularly, to microelectromechanical valves having single crystalline components and an associated fabrication method.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized, optical applications which include MEMS light valves and shutters, and electrical applications which include MEMS relays.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, controlled thermal expansion of an actuator or other MEMS component has been used to actuate MEMS devices. See, for example, U.S. Pat. No. 5,909,078 and U.S. patent application Ser. Nos. 08/936,598 and 08/965,277, all assigned to MCNC, also the assignee of the present invention, which describe MEMS devices having thermally actuated microactuators, the contents of which are incorporated herein by reference.
An example of a thermally actuated microactuator for a MEMS device comprises one or more arched beams extending between a pair of spaced apart supports. Thermal actuation of the microactuator causes further arching of the arched beams which results in useable mechanical force and displacement. The arched beams are generally formed from nickel using a high aspect ratio lithography technique which produces arched beams with aspect ratios up to 5:1. Although formed with high aspect ratio lithography, the actual nickel arched beams have rather modest aspect ratios and may therefore have less out-of-plane stiffness and be less robust than desired in some instances. Further, the lithography technique used to form nickel arched beams may result in the arched beams being spaced fairly far apart, thereby increasing the power required to heat the arched beams by limiting the amount that adjacent arched beams heat one another. In addition, the resulting microactuator may have a larger footprint than desired as a result of the spacing of the arched beams. Thus, there exists a need for arched beams having higher aspect ratios in order to increase the out-of-plane stiffness and the robustness of microactuators for MEMS devices. In addition, there is a desire for microactuators having more closely spaced arched beams to enable more efficient heating and a reduced size.
Nickel microactuators are typically heated indirectly, such as via a polysilicon heater disposed adjacent and underneath the actuator, since direct heating of the nickel structure (such as by passing a current therethrough) is inefficient due to the low resistivity of nickel. However, indirect heating of the microactuator of a MEMS device results in inefficiencies since not all heat is transferred to the microactuator due to the necessary spacing between the microactuator and the heater which causes some of the heat generated by the heater to be lost to the surroundings.
Nickel does have a relatively large coefficient of thermal expansion that facilitates expansion of the arched beams. However, significant energy must still be supplied to generate the heat necessary to cause the desired arching of the nickel arched beams due to the density thereof. As such, although MEMS devices having microactuators with nickel arched beams provide a significant advance over prior actuation techniques, it would still be desirable to develop MEMS devices having microactuators that could be thermally actuated in a more efficient manner in order to limit the requisite input power requirements.
Thermally actuated MEMS valve structures having nickel arched beam actuators typically also have valve plates comprised of nickel. Since the limited aspect ratios attainable with nickel results in structures similarly limited in out-of-plane stiffness and robustness, MEMS valves having nickel valve plates are generally restricted to lower pressure fluid systems in order for the valve to operate with acceptable sealing. While out-of-plane stops for the valve plates may be helpful in increasing the pressure capabilities of a MEMS valve, stops are typically difficult to construct using conventional semiconductor processing techniques for MEMS valves having nickel valve plates. Thus, there exists a further need for more robust MEMS valves with valve plates having increased out-of-plane stiffness and thus for application in higher pressure fluid systems. In addition, it would be desirable for the valve construction to facilitate the formation of out-of-plane stops using conventional semiconductor processing techniques, wherein the stops would contribute to the stability and sealing ability of the valve plate.
The above and other needs are met by the present invention which, in one embodiment, provides a microelectromechanical (MEMS) valve comprising a microelectronic substrate defining at least one opening therethrough, a thermally actuated microactuator disposed thereon and comprised of a single crystalline material, such as silicon, and at least one valve plate comprised of a single crystalline material and having at least one valve seat. The valve plate is operably engaged with the microactuator and adapted to move the valve seat between a disengaged open position and an engaged closed position with respect to the corresponding opening in the microelectronic substrate. More particularly, thermal actuation of the microactuator causes the valve seat to be engaged and/or disengaged with the opening in the substrate as a result of the operable contact between the valve plate and the microactuator.
The microactuator preferably comprises a pair of spaced apart supports disposed on the substrate and at least one arched beam extending therebetween. The microactuator may also include an actuator member that is operably coupled to the at least one arched beam and which extends outwardly therefrom. The microactuator further includes means for heating the at least one arched beam to cause further arching such that the actuator member moves the valve plate between a closed position and an open position with respect to the opening in the substrate. In a closed position, the valve plate is positioned with the valve seat adjacent to the opening in the microelectronic substrate such that the valve seat engages and seals the opening. In an open position, the valve seat is at least partially disengaged from the opening such that the opening is not sealed. Accordingly, the MEMS valve of the present invention may be provided in a normally-closed or a normally-open configuration. Further, the MEMS valve may comprise at least one mechanical stop disposed adjacent the valve plate for providing an out-of-plane restraint for the valve plate, as well as facilitating alignment thereof, with respect to the substrate. In addition, the MEMS valve may also comprise at least one latch disposed on the substrate and capable of interacting with the valve plate, following a predetermined displacement thereof due to thermal actuation of the microactuator, such that the valve plate may be restrained in a predetermined position with respect to the opening without requiring continuous actuation of the microactuator. In some embodiments, the latch may be, for example, thermally actuated or electrostatically actuated.
As will be apparent, each arched beam of the microactuator is comprised of a semiconductor material and has opposed distal portions proximate the respective supports and a medial portion extending between the distal portions. According to one preferred embodiment, a metal layer is disposed on the distal portions of the arched beam, leaving the medial portion of the arched beam substantially free of the metal. Accordingly, an electrical current passing between the supports preferentially heats the medial portion of the arched beam and causes further arching thereof.
Another aspect of the present invention comprises the associated method to form a microelectromechanical valve having single crystalline components. According to one preferred method, a microelectronic substrate is initially formed that defines at least one opening therethrough comprising the valve opening. A first wafer comprised of a single crystalline material then has at least one valve seat formed thereon, either from the same single crystalline material or another suitable material, before being bonded upon a surface of the substrate such that the valve seat is adjacent the substrate. Thereafter, the first wafer is polished to a desired thickness. At least one mechanical stop is then formed adjacent the valve plate for providing an out-of-plane restraint for the valve plate, as well as facilitating alignment thereof, with respect to the substrate. At least one thermally actuated microactuator and at least one valve plate are then formed from the first wafer such that portions of the microactuator and the valve plate are moveable relative to the microelectronic substrate and such that the microactuator is operably engaged with the valve plate. In some embodiments, the microactuator and the valve plate may be at least partially formed from the first wafer prior to the first wafer being bonded to the substrate. Accordingly, thermal actuation of the microactuator causes the microactuator to move the valve plate, and thus the valve seat, between an open position and a closed position with respect to the opening defined by the microelectronic substrate to thereby form the valve. Further, at least one latch may also be formed on the substrate that is capable of interacting with the valve plate, following a predetermined displacement thereof due to thermal actuation of the microactuator, such that the valve plate may be restrained in a predetermined position with respect to the opening.
Thus, a MEMS valve can be formed in accordance with the present invention that includes arched beams and valve plates formed of a single crystalline silicon. Fabricating the arched beams and the valve plates from single crystalline silicon allows these components to be formed with aspect ratios of up to at least 10:1, particularly by using a deep reactive ion etching process. The higher aspect ratios of the arched beams and the valve plates increases their out-of-plane stiffness and constructs a more robust device having greater strength and stiffness. For example, a higher aspect ratio valve plate may accordingly allow operation of a MEMS valve in a higher pressure fluid system. The fabrication techniques of the present invention also permits the arched beams and other components to be more closely spaced. The closer spacing between adjacent silicon arched beams, for example, results in more effective transfer of heat between adjacent arched beams. In addition, the single crystalline silicon microactuator can be directly heated, such as by passing a current therethrough. As will be apparent, direct heating of the microactuator is generally more efficient than indirect heating. Further, although the coefficient of thermal expansion of silicon is less than that of metals, such as nickel, silicon is significantly less dense than nickel such that for a given amount of power, a silicon arched beam can generally be heated more, and therefore arch more, than a corresponding nickel arched beam. Therefore, the MEMS valve of the present invention can have greater out-of-plane stiffness, can be more robust, and the thermally actuated microactuators thereof can be more efficiently and controllably heated than conventional MEMS microactuators having metallic arched beams.